.TH tcpdrop.bt 8  "2018-11-24" "USER COMMANDS"
.SH NAME
tcpdrop.bt \- Trace kernel-based TCP packet drops with details. Uses Linux bpftrace/eBPF
.SH SYNOPSIS
.B tcpdrop.bt
.SH DESCRIPTION
This tool traces TCP packets or segments that were dropped by the kernel, and
shows details from the IP and TCP headers, the socket state, and the
kernel stack trace. This is useful for debugging cases of high kernel drops,
which can cause timer-based retransmits and performance issues.

This tool works using dynamic tracing of the tcp_drop() kernel function,
which requires a recent kernel version.

This tool is limited to ipv4, and cannot parse tcpflags as bpftrace currently cannot parse socket buffers in the way that bcc can.

Since this uses BPF, only the root user can use this tool.
.SH REQUIREMENTS
CONFIG_BPF and bpftrace.
.SH EXAMPLES
.TP
Trace all tcp drops:
#
.B tcpdrop.bt
.TP
.SH FIELDS
.TP
TIME
Time of the call, in HH:MM:SS format.
.TP
PID
Process ID that was on-CPU during the drop. This may be unrelated, as drops
can occur on the receive interrupt and be unrelated to the PID that was
interrupted.
.TP
COMM
Process name
.TP
SADDR:SPORT
Source IP address and source TCP port.
.TP
DADDR:DPORT
Destination IP address and destination TCP port.
.TP
STATE
TCP session state ("ESTABLISHED", etc).
.SH OVERHEAD
This traces the kernel tcp_drop() function, which should be low frequency,
and therefore the overhead of this tool should be negligible.

As always, test and understand this tools overhead for your types of
workloads before production use.
.SH SOURCE
This is from bpftrace
.IP
https://github.com/bpftrace/bpftrace
.PP
Also look in the bpftrace distribution for a companion _examples.txt file
containing example usage, output, and commentary for this tool.

This is a bpftrace version of the bcc tool of the same name. The bcc tool
may provide more options and customizations.
.IP
https://github.com/iovisor/bcc
.SH OS
Linux
.SH STABILITY
Unstable - in development.
.SH AUTHOR
Brendan Gregg, adapted for bpftrace by Dale Hamel
.SH SEE ALSO
tcplife.bt(8), tcpaccept.bt(8), tcpconnect.bt(8), tcptop(8)
